avfilter/xstack: Add support for fixed size grid

Add a short hand parameter for making a fixed size grid. The existing
xstack layout parameter syntax gets tedious if all one wants is a
matrix like grid of the input streams. Add a grid option to the xstack
filter that simplifies this use case by simply specifying the number of
rows and columns instead of specific x/y co-ordinate for each stream.

Also updating the filter documentation to explain the new option.

Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>

doc/filters.texi

@@ -24381,8 +24381,26 @@ the output video frame will be filled. Similarly, videos can overlap each
 other if their position doesn't leave enough space for the full frame of
 adjoining videos.
 
-For 2 inputs, a default layout of @code{0_0|w0_0} is set. In all other cases,
-a layout must be set by the user.
+For 2 inputs, a default layout of @code{0_0|w0_0} (equivalent to
+@code{grid=2x1}) is set. In all other cases, a layout or a grid must be set by
+the user. Either @code{grid} or @code{layout} can be specified at a time.
+Specifying both will result in an error.
+
+@item grid
+Specify a fixed size grid of inputs.
+This option is used to create a fixed size grid of the input streams. Set the
+grid size in the form @code{COLUMNSxROWS}. There must be @code{ROWS * COLUMNS}
+input streams and they will be arranged as a grid with @code{ROWS} rows and
+@code{COLUMNS} columns. When using this option, each input stream within a row
+must have the same height and all the rows must have the same width.
+
+If @code{grid} is set, then @code{inputs} option is ignored and is implicitly
+set to @code{ROWS * COLUMNS}.
+
+For 2 inputs, a default grid of @code{2x1} (equivalent to
+@code{layout=0_0|w0_0}) is set. In all other cases, a layout or a grid must be
+set by the user. Either @code{grid} or @code{layout} can be specified at a time.
+Specifying both will result in an error.
 
 @item shortest
 If set to 1, force the output to terminate when the shortest input

libavfilter/vf_stack.c

@@ -48,6 +48,8 @@ typedef struct StackContext {
     int is_vertical;
     int is_horizontal;
     int nb_planes;
+    int nb_grid_columns;
+    int nb_grid_rows;
     uint8_t fillcolor[4];
     char *fillcolor_str;
     int fillcolor_enable;
@@ -85,33 +87,42 @@ static av_cold int init(AVFilterContext *ctx)
     if (!strcmp(ctx->filter->name, "hstack"))
         s->is_horizontal = 1;
 
-    s->frames = av_calloc(s->nb_inputs, sizeof(*s->frames));
-    if (!s->frames)
-        return AVERROR(ENOMEM);
-
-    s->items = av_calloc(s->nb_inputs, sizeof(*s->items));
-    if (!s->items)
-        return AVERROR(ENOMEM);
-
     if (!strcmp(ctx->filter->name, "xstack")) {
+        int is_grid;
         if (strcmp(s->fillcolor_str, "none") &&
             av_parse_color(s->fillcolor, s->fillcolor_str, -1, ctx) >= 0) {
             s->fillcolor_enable = 1;
         } else {
             s->fillcolor_enable = 0;
         }
-        if (!s->layout) {
+        is_grid = s->nb_grid_rows && s->nb_grid_columns;
+        if (s->layout && is_grid) {
+            av_log(ctx, AV_LOG_ERROR, "Both layout and grid were specified. Only one is allowed.\n");
+            return AVERROR(EINVAL);
+        }
+        if (!s->layout && !is_grid) {
             if (s->nb_inputs == 2) {
-                s->layout = av_strdup("0_0|w0_0");
-                if (!s->layout)
-                    return AVERROR(ENOMEM);
+                s->nb_grid_rows = 1;
+                s->nb_grid_columns = 2;
+                is_grid = 1;
             } else {
-                av_log(ctx, AV_LOG_ERROR, "No layout specified.\n");
+                av_log(ctx, AV_LOG_ERROR, "No layout or grid specified.\n");
                 return AVERROR(EINVAL);
             }
         }
+
+        if (is_grid)
+            s->nb_inputs = s->nb_grid_rows * s->nb_grid_columns;
     }
 
+    s->frames = av_calloc(s->nb_inputs, sizeof(*s->frames));
+    if (!s->frames)
+        return AVERROR(ENOMEM);
+
+    s->items = av_calloc(s->nb_inputs, sizeof(*s->items));
+    if (!s->items)
+        return AVERROR(ENOMEM);
+
     for (i = 0; i < s->nb_inputs; i++) {
         AVFilterPad pad = { 0 };
 
@@ -244,6 +255,48 @@ static int config_output(AVFilterLink *outlink)
                 width += ctx->inputs[i]->w;
             }
         }
+    } else if (s->nb_grid_rows && s->nb_grid_columns) {
+        int inw = 0, inh = 0;
+        int k = 0;
+        int row_height;
+        height = 0;
+        width = 0;
+        for (i = 0; i < s->nb_grid_rows; i++, inh += row_height) {
+            row_height = ctx->inputs[i * s->nb_grid_columns]->h;
+            inw = 0;
+            for (int j = 0; j < s->nb_grid_columns; j++, k++) {
+                AVFilterLink *inlink = ctx->inputs[k];
+                StackItem *item = &s->items[k];
+
+                if (ctx->inputs[k]->h != row_height) {
+                    av_log(ctx, AV_LOG_ERROR, "Input %d height %d does not match current row's height %d.\n",
+                           k, ctx->inputs[k]->h, row_height);
+                    return AVERROR(EINVAL);
+                }
+
+                if ((ret = av_image_fill_linesizes(item->linesize, inlink->format, inlink->w)) < 0) {
+                    return ret;
+                }
+
+                item->height[1] = item->height[2] = AV_CEIL_RSHIFT(inlink->h, s->desc->log2_chroma_h);
+                item->height[0] = item->height[3] = inlink->h;
+
+                if ((ret = av_image_fill_linesizes(item->x, inlink->format, inw)) < 0) {
+                    return ret;
+                }
+
+                item->y[1] = item->y[2] = AV_CEIL_RSHIFT(inh, s->desc->log2_chroma_h);
+                item->y[0] = item->y[3] = inh;
+                inw += ctx->inputs[k]->w;
+            }
+            height += row_height;
+            if (!i)
+                width = inw;
+            if (i && width != inw) {
+                av_log(ctx, AV_LOG_ERROR, "Row %d width %d does not match previous row width %d.\n", i, inw, width);
+                return AVERROR(EINVAL);
+            }
+        }
     } else {
         char *arg, *p = s->layout, *saveptr = NULL;
         char *arg2, *p2, *saveptr2 = NULL;
@@ -436,6 +489,7 @@ const AVFilter ff_vf_vstack = {
 static const AVOption xstack_options[] = {
     { "inputs", "set number of inputs", OFFSET(nb_inputs), AV_OPT_TYPE_INT, {.i64=2}, 2, INT_MAX, .flags = FLAGS },
     { "layout", "set custom layout", OFFSET(layout), AV_OPT_TYPE_STRING, {.str=NULL}, 0, 0, .flags = FLAGS },
+    { "grid", "set fixed size grid layout", OFFSET(nb_grid_columns), AV_OPT_TYPE_IMAGE_SIZE, {.str=NULL}, 0, 0, .flags = FLAGS },
     { "shortest", "force termination when the shortest input terminates", OFFSET(shortest), AV_OPT_TYPE_BOOL, {.i64=0}, 0, 1, .flags = FLAGS },
     { "fill",  "set the color for unused pixels", OFFSET(fillcolor_str), AV_OPT_TYPE_STRING, {.str = "none"}, .flags = FLAGS },
     { NULL },